The two important factors are power (or energy production rate), measured in watts, and total energy produced, measured in watt-hours.
We know that about 18 kilowatt hour (kWh) per person per day is the average electrical energy used so for half the people of Scotland this would come to 2.5 x 18 gigawatt hour (gWh) per day. The projected maximum for Pentland is 1.9 x 24 gWh per day so, yes, a close match. But currently half of Scotland’s electricity production capacity is about 6 x 24 gWh per day, or three times as much. Why the discrepancy? The answer is that we do not use electricity at an average rate.
I do not have figures but let’s say that peak winter usage rate is at least three times average, then of course we will require a three times greater power source, so the Pentland Firth scheme could possibly fulfil the electricity needs of only one-sixth of the population.
Also we have to remember that electricity is only about one quarter of our internal energy requirements so we are back to that 20-fold exaggeration. Plus ça change!
(Dr) A McCormick